Optimization of power deposition and a heating strategy for external ultrasound thermal therapy

Abstract

The purpose of this paper is to examine the thermal dose distribution, to configure the optimal absorbed power deposition, and to design an appropriate heating strategy for ultrasound thermal therapy. This work employs simulation programs, which are based on the transient bio-heat transfer equation and an ideal absorbed power deposition or an ideal temperature elevation within a cube of tissue, to study the optimal absorbed power deposition. Meanwhile, a simplified model of a scanned ultrasound transducer power deposition (a cone with convergent/divergent shape) is used to investigate the heating strategy for a large tumor with a sequence of heating pulses. The distribution of thermal dose equivalence defined by Sapareto and Dewey is used to evaluate the heating result for a set of given parameters. The parameters considered are the absorbed power density, heating duration, temperature elevation, blood perfusion, and the size of heating cube. The results demonstrate that the peak temperature is the key factor determining the thermal dose for this short-duration heating. Heat conduction has a very strong influence on the responses of temperature and thermal dose for a small heating cube and the boundary portion of a large heating cube. Hence, for obtaining the same therapeutic result, a higher power density is required for these two conditions to compensate the great temperature difference between the heating cube and the surrounding tissue. The influence of blood perfusion on the thermal dose is negligible on the boundary portion of the heating cube, while in the central portion it may become a crucial factor as a lower power density is used in this portion to save the delivered energy. When using external ultrasound heating method to treat a large tumor, the size of heating unit, the sequence of heating pulses, and the cooling-time interval between the consecutive heating pulses are the important factors to be determined to have an appropriate treatment within a reasonable overall treatment time.

Pilot point temperature regulation for thermal lesion control during ultrasound thermal therapy

“…This observation is consistent with the conclusions in our previous works (LIN et al, 2001;LIU et al, 2003).…”

“…This is because the regulated temperature of 52.5°C for pattern E was relatively higher than the required level (compared with 49°C), and the effect of blood sub-region's central axis. Between the heating of two different sub-regions, a cooling time interval was introduced to let the temperatures in the field all decay to below 43°C and prevent near-field overheating caused by thermal dose accumulation (LIN et al, 2001).…”

“…11, was designed to be able to dissipate the heat energy more easily and to shorten the introduced cooling time interval (LIN et al, 2001). Fig.…”

“…First, the accumulation ofthelmal dose is a function of temperature and time and is highly non-linear (SAPARETO and DEWEY, 1984). Secondly, after the heating power is turned off, the thermal dose keeps accumulating, and this accumulation depends on different positions, blood perfusion rates and heating strategies (LIN et al, 2001). For these two reasons, previous studies only focused on the issues of off-line optimisation of the thelmal lesion formation (HUTCHINSON and HYNYNEN, 1996;DAUM and HYNYNEN, 1999;WAN et al, 1999;LIU et al, 2003).…”

A robust power deposition scheme for tumors with large counter-current blood vessels during hyperthermia treatment

“…Others evaluated high intensity focused ultrasound (HIFU) treatment time reductions using the same model to simulate treatments of homogeneous constant property tumors of various simple generic shapes and sizes [8]. The amplitude and phase of every applicator transducer are important parameters to tune for HIFU treatment in order to optimize the power density distribution or the resulting temperature distribution [9,10].…”